The invention concerns a spun-bonded non-woven from polyolefin filaments with a filament titer <1.6 dtex. The spun-bonded non-woven is characterized by special mechanical properties.
The invention also concerns production of a laminate, using the spun-bonded non-woven according to the invention, as well as the use of the spun-bonded non-woven and use of the laminate produced with the spun-bonded non-woven.
Non-wovens are textile fabrics that can be produced in different ways. In addition to wet non-woven production and dry non-woven production, a distinction is made between melt spinning and melt blowing (melt-blown technology). The two technologies melt spinning and melt blowing have the advantage that the plastic granulate can be converted directly to the finished fabric by means of a corresponding installation. The comparatively high productivity of these installations during non-woven production is based on this.
In melt spinning, polymer granulates are melted in an extruder, forced through the openings, so-called spinnerets, of a spinning device and pneumatically or mechanically stretched after cooling. The ultimate strength of the filaments is established by the stretching process. The filaments, loosely laid after stretching on a moving laying belt, are chemically or thermally compacted to so-called bonding points in the area of the intersection points in contact. The softness of the non-woven so formed then diminishes with increasing compaction, during which its bending rigidity increases. Several identical or different overlapping spun-bonded layers can be compacted thermally, for example, by calandering, into a composite material (laminate).
During melt blowing, the productivity is lower than in melt spinning. An additional factor is that non-wovens produced by melt blowing have lower mechanical loadability than those produced by melt spinning. However, the non-wovens produced by melt blowing are characterized by very good barrier properties.
The objective of low-cost non-woven production is therefore replacement (or in the case of production of a laminate, reduction) of non-wovens produced by melt blowing by non-wovens that were ideally produced completely by melt spinning.
The properties of a spun-bonded non-woven are comprehensively described by the basis weight and density, as well as the mechanical properties, like maximum tensile force and maximum tensile elongation, and also the barrier properties, for example, water-tightness and air permeability.
The basis weight of a spun-bonded non-woven states its weight as a function of surface in g/m2, in which the density of a spun-bonded non-woven corresponds to the ratio of basis weight and thickness of the spun-bonded non-woven. The reduction in basis weight of the spun-bonded non-woven can therefore be achieved either by reducing the spun-bonded non-woven density or reducing the spun-bonded non-woven thickness. In the normal case and when all other production parameters are kept constant, however, both of these are at the expense of mechanical properties, and also at the expense of the barrier properties of the spun-bonded non-woven.
The reduction of basis weight, however, is a central parameter in product improvement, because it essentially co-determines the wear comfort of products produced from the non-woven. Thus, a continuous trend toward lighter weight spun-bonded non-wovens can be seen in baby diapers, incontinence products and in products for feminine hygiene. On the other hand, however, air permeability diminishes with increasing non-woven thickness. But it is precisely these products that simultaneously require a guarantee of mechanical properties and barrier properties even with reduced basis weight. Basis weight, mechanical properties and barrier properties of the spun-bonded non-woven, however, depend on different parameters. A critical parameter that determines all the mentioned quantities is filament titer. The filament titer of a yarn or filament is stated as weight referred to length and describes its fineness. A high yarn fineness then means a smaller weight/length ratio. The yarn fineness is measured in tex, in which 1 tex is 1 gram per 1000 m, and a decitex (dtex) corresponds to 1 gram per 10,000 m.
Lower spun-bonded non-woven thicknesses are accessible, in principle, by using filaments with a lower filament titer, since finer filaments, when total throughput remains the same with unaltered speed of the conveyor belt during laying for non-woven formation, produce a non-woven layer of lower thickness with generally higher density, because of their smaller diameter.
When ordinary melt spinning technologies are used (U.S. Pat. No. 3,692,618, U.S. Pat. No. 5,032,329, U.S. Pat. No. 5,814,349, WO03038174 or WO02063087), finer filaments are produced by the fact that the polymer throughput (in grams of polymer per minute and hole) is reduced. This approach, however, is connected with a reduction in total installation throughput and is therefore undesired with respect to productivity. On the other hand, an increase in total throughput, when the other production parameters are kept constant, generally leads to thickening of the filaments and therefore to an increase in filament titer. An increase in filament titer, however, is not desired with respect to the objective of the present invention, which consists of producing a lightweight spun-bonded non-woven.
If, according to DE 10360845 A1, a spinning device with a spinning plate, which has a spinning device with a significantly increased number of nozzle openings per meter of spinning plate, is used to produce the filaments forming the lightweight spun-bonded non-woven, the polymer throughput in grams per unit of time and per hole is reduced, but the total throughput overall remains unchanged. At the same time, finer filaments are obtained, which permit the production of lighter-weight spun-bonded non-wovens.
A general problem of the spun-bonded non-wovens with lower weight known from the prior art, as already mentioned, is the lower mechanical stability. Such spun-bonded non-wovens can be easily torn, especially across the machine direction, and have limited dimensional stability. In the case of WO99/32699, which discloses a spun-bonded non-woven with a basis weight of about 13.6 g/m2, this drawback is overcome with additional expense by reinforced bonding and a special pattern of the engraving rolls during thermal bonding.
Multilayer composite non-wovens (laminates) are also known from the prior art, whose outer layers consist of melt-spun spun-bonded non-woven layers, whereas at least one of the inner layers consists of very fine fibers, preferably produced by melt blowing. The low mechanical loadability of the layers produced by melt blowing makes the outer spun-bonded non-woven layers produced by melt spinning absolutely essential, in order to give the composite non-woven good mechanical loadability overall. Production of spun-bonded non-wovens with the best possible combination of barrier properties and mechanical properties has therefore only been guaranteed thus far by producing such laminates.
Spun-bonded non-wovens that are produced only by melt spinning are also known from the prior art, in which filaments with low filament titer are used. Thus, U.S. Pat. No. 5,885,909 discloses a non-woven made of polyolefin, constructed from fibers with a filament titer of only 0.33 dtex, which is characterized by improved barrier properties and breathing properties. The basis weight, however, is comparatively high at ≦44.1 g/m2 with a non-woven thickness of 0.33 mm and a density of 0.1336 g/m3.
An alternative method for production of lighter-weight spun-bonded non-wovens from olefin polymers is described in US 2004/0070101. The filaments produced by extrusion have an “island-in-the-sea” structure, in which the sea polymer has different solution properties than the island polymer and is removed after non-woven production by dissolving it out from the non-woven.